Steam turbines, particularly in the low pressure range, are equipped with rotor blades of long length which, if no suitable countermeasures are adopted, are prone to undesirable vibrations during operation. A countermeasure is to provide a mechanical connection between the blade airfoils of the rotor blades in the region of the blade tip, as described in DE 102008059836A1, for example.
Such connections may be made by means of special stabilizing elements in a center region of the blade airfoils, as is described in printed publication U.S. Pat. No. 4,257,743.
In FIG. 1, a connection is shown in a detail. The rotor blades 11 of a steam turbine 10 there have in each case a through-hole 13, through which an arc-shaped stabilizing element 12 is inserted and soldered with hard solder, the shape of the stabilizing element being shown in FIG. 2 in plan view from above (FIG. 2a) and in side view (FIG. 2b).
The stabilizing elements 12 of FIGS. 1 and 2 are all of a similar design. They have in each case two end sections 18 and 19. The first end section 18 is formed as V-shaped groove (also referred to as a “notch”) with two groove walls 20, and the second end section 19, which matches it, is formed as a V-shape wedge with inclined walls 22 (also referred to as an “iron sight”). Arranged between the end sections 18, 19, in the middle, is a thickened center piece 15 from which arms 16, 17 extend to the end sections 18, 19. The stabilizing elements 12, which are arranged one behind the other in the circumferential direction, engage in each case by their second end section 19 in the first end section 18 of the subsequent stabilizing element, as is shown by way of example in FIG. 3. In this way, an engagement section 14, which is marked by means of the dashed circle in FIG. 3, is created between two consecutive stabilizing elements 12a, 12b. The engagement section 14 is delimited on the oppositely-disposed outer sides by means of two planar outer surfaces 21. Since the blade is twisted, the iron sight always buts against the notch on one side only (installed side). If the blade untwists during operation, the iron sight buts against the notch by the other side (operating side).
In the case of the installed stabilizing elements 12 or 12a, 12b cracks may occur in the engagement section 14 during operation, the cracks occurring mainly on the notch side, i.e. in the groove walls 20 of the first end sections 18, and therefore are not visible from the outside in the first instance.
In principle, a crack inspection of the engagement section 14 by MPI methods (Magnetic Particle Inspection) may be undertaken. These methods, however, have the disadvantage that cracks can be detected only if they occur on the outside, which as a rule is too late in order to be able to exchange defective elements in good time (major consequential damage).
It would also be conceivable to inspect the engagement section 14 manually by means of ultrasound. Such a manual inspection could certainly detect cracks earlier, but on account of the confined space conditions between the rotor blades 11 is difficult, time-consuming and not very reliable.